Method for detecting steam leakage in a heat exchanger having circulation tubes surrounded by liquid sodium and devices for the application of said method

ABSTRACT

THE LEAK FLOW RATE, IN PARTICULAR FOR THE DETERMINATION OF A MICRO-LEAK, IN HEAT EXCHANGERS EMPLOYED IN CONJUNCTION WITH FAST NUCLEAR REACTORS WHICH ARE COOLED BY LIQUID SODIUM IS DETERMINED EMPLOYING KNOWN MEANS BY LOCATING A GIVEN GROUP OF TUBES INCLUDING THE TUBE WHICH EXHIBITS LEAKAGE AMONG ALL THE TUBES OF A HEAT EXCHANGER WITHOUT MODIFYING THE OPERATING CONDITIONS OF THE INSTALLATION. THE TUBE IN THE GROUP IN WHICH A LEAK HAS DEVELOPED IS LOCATED. THE DEFECTIVE TUBE IS THEM PLUGGED OUTSIDE THE TANK WITHOUT DRAINING THE SODIUM CONTAINED THEREIN AND WITHOUT CONNECTING THE STREAM CIRCULATION TUBES TO THE ATMOSPHERE.   D R A W I N G

March 20, 1-973 A. BRACHET ETAL 3,721,116

METHOD FOR DETECTING STEAM LEAKAGE IN A HEAT EXCHANGER HAVINGCIRCULATION TUBES SURROUNDED BY LIQUID SODIUM AND DEVICES FOR THEAPPLICATION OF SAID METHOD Filed Dec. 1, 1970 2 SbeatsPSheet 1 March 20,1973 A. BRACHET ETAL 3.721.

METHOD FOR DETECTING STEAM LEAKAGE IN A HEAT EXCHANGER HAVINGCIRCULATION TUBES SURROUNDED BY LIQUID SODIUM AND DEVICES FOR v THEAPPLICATION OF SAID METHOD Filed Dec. 1, 1970 2 SheetseSheet 2 FIG. 6

United States Patent 3,721,116 METHOD FOR DETECTING STEAM LEAKAGE IN AHEAT EXCHANGER HAVING CIRCULATIDN TUBES SURROUNDED BY LIQUID SODIUM ANDDEVICES FOR THE APPLICATION OF SAID METHOD Alain Bracllet, Orsay, andLouis Lannou, Montessori,

France, assignors to Electricite de France (Service National), Paris,France Filed Dec. 1, 1970, Ser. No. 93,940 Claims priority, applicationFrance, Dec. 3, 1969, 6941746 Int. Cl. G01m 3/04 US. Cl. 73-40 ClaimsABSTRACT 0F THE DISCLOSURE The leak flow rate, in particular for thedetermination of a micro-leak, in heat exchangers employed inconjunction with fast nuclear reactors which are cooled by liquid sodiumis determined employing known means by locating a given group of tubesincluding the tube which exhibits leakage among all the tubes of a heatexchanger without modifying the operating conditions of theinstallation. The tube in the group in which a leak has developed islocated. The defective tube is then plugged outside the tank withoutdraining the sodium contained therein and Without connecting the steamcirculation tubes to the atmosphere.

This invention relates to a method for locating a tube in which a leakhas developed among all the tubes of a heat exchanger, especially inwhich liquid sodium is employed as primary fluid and circulated incontact with the external walls of said tubes in order to transfer heatwhile the secondary fluid which absorbs said heat is circulated withinthe interior of the tubes is water in the state of steam or of liquidmixed with steam.

In a heat exchanger of this type consisting of a very large number oftubes which pass in leak-tight manner through a tank containing theliquid sodium and are con nected externally of said tank to themanifolds which effect the distribution and circulation of the water orsteam within said tubes, the aim of this invention is to minimize outageof the installation in the event of detection of a leak in any tube,detection being carried out by determining the quantity of hydrogenproduced by the reaction of the water with the sodium, and to select theleaking tube with accuracy but without any need to drain the sodium fromthe tank and preferably even without discharging the steam circuit tothe atmosphere if this is permitted by the constructional design of theheat exchanger. A further object of the invention is to make it possiblein a simple manner, when the defective tube has been located, to plugsaid tube at both ends, locating and plugging operations being carriedout in a simple and rapid manner, thereby permitting start-up of theinstallation in a very short time.

It is known that heat exchangers and especially those employed inconjunction with nuclear reactors which are cooled with liquid sodiumgive rise to awkward problems in regard to safety of installations. Infact, as a result of any leakage which occurs in one of theheat-exchanger tubes, the water in the form of either liquid or steamwhich is circulated within said tube at high pressure is permitted topenetrate into the sodium which is in contact with the external wall ofsaid tube. The resultant reaction between the sodium and the water givesrise to many disadvantages which lead ultimately to outage of theinstallation over a more or less extended period of time.

If leakage of the tube considered is substantial and 3,7Zl,ll6 PatentedMar. 20, 1973 higher, for example, than a few grams of water per secondand in particular When said leakage results from fracture of said tube,shut-down of the installation is necessary and reconditioning is alwaysa complex and costly operation. If the leakage is relatively minor andranges, for example, from a few hundred grams per second to a few gramsper second resulting in particular from a crack or hole of small size,the reaction is less violent; but experience shows that a leak of thistype nevertheless behaves as a vertiable high-temperature jet in aliquid medium, said jet being liable to cause serious and rapid damageto the adjacent structures and in particular to the other tubes. In thiscase also, total shut-down of the installation is necessary in practicein order to carry out esesntial inspections and repairs.

Finally, if the leakage of any one of the tubes of the bundle whichconstitutes the heat exchanger is extremely small and less than a fewhundredths of a gram per second, for example, and can consequently beassimilated to what is commonly referred-to as a micro-leak arising inparticular from a manufacturing defect which has escaped preliminaryinspection of the tubes prior to assembly within the heat exchanger,from a crack resulting from fatigue stress or any other cause, it isfound that the consequences in the immediate vicinity are relativelynonexistent or at least sufliciently minor to produce appreciable damageonly after a substantial time of action; in the case last mentioned, itis therefore possible to contemplate very limited outage of theinstallation. In fact, if the tube which has a micro-leak can beidentified and put out of service so as to introduce minimum disturbancein the operation of the installation, complete disassembly of theapparatus for inspection of all the tubes of this later is not necessarybut is on the contrary essential in the case of leaks having a higherflow rate by reason of their action on the immediate vicinity.

To this end, the present invention relates to a method for carrying outthe repair of a minimum leak or microleak within the meaning definedabove without calling for extended outage of the installation, thedefective tube being readily plugged after location, that is to sayplugged at both ends outside the tank which contains the sodium. Saidplugging results only in slight reduction in thermal efficiency of theheat exchanger, this reduction being usually negligible when taking intoaccount the very large number of tubes constituting the installation.

To this end, the method according to the invention which consists as aninitial step without modifying the pressure and temperature regime undernormal operating conditions of the heat exchanger in locating andclassifying the leakage in the category of micro-leaks by detection ofhydrogen in the sodium by means of a known method and in selecting fromall the heat-exchanger tubes a group of tubes containing the particulartube in which a micro-leak has developed. Said method essentiallyconsists in bringing the heat exchanger to an isothermal temperature inwhich the thermal power exchanged between the sodium and the water issubstantially zero, in simultaneously depressurizing the steam withinall the tubes of the heat exchanger to a pressure which is slightlyhigher than the pressure of the sodium, in connecting each tube of thegroup successively to a reservoir containing hydrogenated fluid which isintroduced into said tubes at a pressure equal at a maximum to thepressure of the steam under normal operating conditions and to theisothermal temperature of the sodium, in detecting the presence ofhydrogen in the sodium by means of the known method for selecting thetube in which a leak has developed and in plugging said tube externallyof the tank.

In a general manner, the known method of detection of hydrogen consistsin determining the quantity of hydrogen contained in the sodium afterdiffusion through a metallic membrane associated with an analyticaldevice such as a mass spectrometer (the method just referred-to and thecorresponding equipment being described in partlcular in report ANL.7520entitled Proceedings of the International Conference on SodiumTechnology and Large Fast Reactor Design Nov. 7-9, 1968 and especiallyin the article entitled Detection of Small Leaks by HydrogenMeasurements in Sodium-heated Steam Generator pp. 345 to 373).

It has already been found uneconomical to make use of conventionalmonitoring and measurement systems such as those described in theabove-mentioned document and employed for total detection of thepresence of hydrogen in order to carry out individual ident1ficat1on oftubes in which leakage has developed during normal operation of the heatexchanger by adapting these systems in particular to a complex mechanismfor tube-bytube inspection of the entire tube bank. A heat exchanger fora high-power nuclear reactor in fact comprises several hundreds or eventhousands of tubes and any method of direct location of individual tubescan rapidly lead to prohibitive capital expenditure.

On the other hand and in accordance with the invention, it becomespossible at relatively low cost to provide a device wherein a group oftubes which may include a tube having a micro-leak has been selected ina first stage by means of any suitable method and as a function both ofthe basic concept and structural design of the heat exchanger, whereinsaid preliminary selection and detection operations are carried outsimultaneously during normal operation of the installation and whereinsaid device carries out accurate identification of the tube whichexhibits leakage among those of the selected group in a second stage oflimited duration during which particular conditions of temperatures andpressures within the heat exchanger are satisfied, said operation beingcarried out with a minimum disturbance of the operation of theinstallation.

After final identification of the group of tubes containing the tubewhich exhibits leakage, the mode of operation which is adopted calls foradjustment of the reactor power in order to bring the sodium within theheat exchanger to a substantially isothermal temperature in which thethermal power transferred through the tubes is practically zero; thistemperature is evidently compatible with the mechanical strength of theapparatus as a whole and with the general operation of the installationwhile being preferentially higher than the melting temperature of sodiumhydroxide (NaOH) which is in the vicinity of 320 C. or more generallyhigher than the temperature of incipient appearance of impurities whichare liable to obstruct the leak from the exterior of the tube. At thesame time, the pressure of the steam within the heat exchanger tubes isreduced to a slightly higher value than that of the sodium in order toprevent penetration of this latter through the tube leak which it issought to detect. During the following stage, the pressure within thetubes of the selected group can be maintained by means of an inert gassuch as argon or nitrogen which is supplied from an auxiliary externalsource. Said intermediate inert gas is intended to replace the steamwithin the tubes of the group under consideration and has the advantageof preventing corrosion of the tubes as well as facilitating thenecessary handling operations. The pres sure of the inert gas is eitherequal to or higher than the steam pressure in the previous stage.

The final stage then consists in injecting a hydrogenated fluid throughan opening which is specially provided in the tubes or through the tubeends and by producing action on each tube in turn, said fluid beingconstituted, for example, by demineralized and degassed water in theliquid or vapor state or by hydrogen or even by any other fluid ormixture of fluids which have the property of liberating hydrogen as aresult of chemical reaction with the sodium. Said fluid is introduced atthe pressure of the inert gas and is accordingly intended to replacethis latter in each tube in turn. In the case of water, the temperatureof this latter is equal to the isothermal temperature of the sodium.When the tube in which a leak has developed is thus put under pressureof hydrogenated fluid, the hydrogen which escapes through the leak isdistributed within the external sodium. The response of the hydrogendetection system then makes it possible to locate said tube immediately.During the final stage, it may prove desirable to reduce the flow rateof sodium within the heat exchanger in order to increase the detectionsensitivity and bring the pressure of the hydrogenated fluid within eachtube in succession to the nominal steam pressure in normal operation sothat the leak should be placed under the same pressure conditions asthose which prevail during said operation. Moreover, it should thus benoted that there is little reason to expect any accidental stopping ofthe leak by the sodium or by the reaction products since all theoperations are carried out in such a manner as to modify the operatingconditions to the least possible extent with respect to the normalconditions during which the leak was initially detected.

When the tube in which a leak is present has thus been determined, thesteam tube circuit which is placed under inert gas pressure isdepressurized to a value which is slightly higher than atmosphericpressure in order to prevent admissions of air during the finaloperation involving plugging of the faulty tube and disconnecting thislatter from the circuit. Once the plugging operation has been completed,the heat exchanger is put back into service without having drained thesodium and even without having totally interrupted the sodiumcirculation at any moment. Furthermore, in the alternative embodiment inwhich liquid water is employed as tracer fluid, it is possible toprevent any admission of air into the steam circuit. In the otheralternative embodiments, the air admission always remains as low aspossible. In consequence, outage of the installation is on the whole ofvery short duration and the operation involving identification of theleaking tube does not in itself involve any substantial disassembly. Inaddition, this operation does not call for special equipment or skilledpersonnel, only the monitoring equipment which is already installedbeing employed for carrying out the detection of hydrogen within thesodium in known manner.

Further properties of the method under consideration will becomeapparent from the following description of a number of exemplifiedembodiments of devices for the practical application of said method,said devices being given by way of non-limitative indication and formingpart of the present invention.

-In the accompanying drawings:

FIGS. 1 and 2 are diagrammatic sectional views of a heat exchangercomprising a tank and an assembly of steam circulation tubes whichare'mounted through said tank;

FIG. 3 is a sectional view of an equipment unit comprising an externalreservoir for putting the different tubes forming part of anypreselected group under pressure of a hydrogenated fluid and possibly ofan inert gas;

FIGS. 4 and 5 are detail views on a larger scale showing a part of oneof the heat-exchanger tubes;

FIG. 6 is a sectional view of another embodiment of a heat exchangerwhich makes use of an alternative form of the method according to theinvention.

In FIGS. 1 and 2, the heat exchanger which is illustrated comprises in avery diagrammatic manner a tank 1 which is adapted to the continuouscirculation of a primary fluid and especially liquid sodium 2 which isderived either directly or indirectly from the core of a nuclear reactor(not shown in the drawings), said sodium being intended to penetrateinto the tank 1 through a pipe 3 and to be discharged through a pipe 4.The circulating sodium is thus in contact with the external surface of aseries of tubes such as the tube which pass in leak-tight manner throughthe tank 1, said tubes being connected externally of the tank to aninput manifold 6 and to an output manifold 7 which serves to circulatewithin said tubes a secondary fluid and especially water in the state ofliquid and/or steam for exchanging heat with the sodium. The examplewhich is shown diagrammatically in FIG. 1 illustrates a particularassembly of the tubes 5 in which these latter are suspended Within thetank 1 while the example illustrated in FIG. 2 shows said tubes whichpass through the tank substantially axially; this last-mentionedarrangement of the so-called drainable tube type makes it possible inparticular to purge said tubes.

FIG. 3 illustrates an external reservoir which makes it possible inaccordance with the method of the invention to inject into eachheat-exchanger tube a suitable quantity of hydrogenated fluid and inparticular demineralized and degassed water 12 in the state of liquid orsteam under the given conditions of temperature and pressure which willbe specified hereinafter. Said reservoir 10 comprises mainly a shell 11within which the depth of water is measured by means of a suitabledevice consisting of an external gage 13, for example, which isconnected to the shell 11 by means of pipes 14 and 15. Said shell issurrounded externally by a suitable layer 16 of heatinsulating materialand provided at the lower portion thereof with a heating element 17 forbringing the water 12 to a given temperature as measured by means of thetemperature transducer 26. The reservoir 10 is also connected to apressurization circuit comprising pipes 18, 19 and 20 which areconnected to a pipe 21 for supplying an inert gas such as argon ornitrogen, pressure gages 22 and 23 being employed for measuring thepressure of said gas and of the water within the reservoir. A va ve 24serves to connect the reservoir 10 to a pipe for the make-up supply ofwater; finall'y, discharge from the reservoir is carried out through avalve 25 which serves to connect said reservoir to any one of theheat-exchanger tubes 5 in accordance with either FIG. 1 or FIG. 2.

In order to permit of connection between the reservoir 10 and the tubes5, said tubes are preferably provided with a lateral opening as shownmore especially in the enlarged views of FIGS. 4 and 5, said openingbeing normally closed by a plug 31 which compresses a seal 32. When thereservoir 10 is to be connected to the tubes 5, the plug 31 is removedand the extremity of the delivery-pipe 33 of the valve 25 is fitted inthe opening 30, said pipe 33 being preferably provided with a ballvalve34 which is controlled by a spring 35 and performs the function of acheck-valve so as to prevent return flow to the reservoir 10 or to theatmosphere.

The method of monitoring and measurement is carried out in the mannerwhich has already been explained in detail. After a group of tubesincluding the tube which exhibits a leak has been identified among allthe heatexchanger tubes 5 in an initial stage and during normaloperation of the installation, said leak having previously been detectedby an ancillary measuring assembly of the diffusion-membrane type (notillustrated) for detecting hydrogen in the sodium and having beenclassified according to the value of flow rate in the category ofmicro-leaks which do not require complete shut-down of the installation,the conditions of isothermal temperature and of pressure in accordancewith the characteristics already mentioned are established within saidinstallation. The steam circuit is then put under a pressure of inertgas corresponding to a value which is slightly higher than that of thesodium, whereupon the devices of the type shown in FIG. 5 are adaptedsuccessively to each tube of the selected group. In the case of a heatexchanger of the type illustrated in FIG. 2, it is also necessary toform a leak-tight separation between the device according to FIG. 5 andthe bottom manifold 6 in order to prevent tubes other than that on whichit is desired to operate from being filled as a result of acommunicatingvessel effect. This precaution is optional in the case ofthe heat exchanger of FIG. 1 subject to the condition that the manifolds6 and 7 are located substantially in the same horizontal plane. If thisis not the case and so far as the heat exchanger according to FIG. 2 isconcerned, it is preferable in order to provide said separation to makeuse of a stability diaphragm as normally fitted at the inlet of eachtube and constituted by an orifice having a sutficient pressure drop toensure that the flow rate to all the heat-exchanger tubes issubstantially constant and uniform and to replace said orifice with afull diaphragm. Whatever form of device may be employed, it is in factadvisable to ensure that said device is capable of carrying out thethree diaphragm functions of damping hydrodynamic instabilities of theheat exchanger, of providing leak-tightness in the direction of thebottom manifold and of providing a nozzle for the injection of thetracer fluid.

When this operation has been completed, the pressure of inert gas withinthe exchanger is increased to a value such that, in the case of theisothermal temperature to which the sodium has previously been brought,the demineralized water which performs the function of hydrogenatedfluid remains in the liquid state after injection into each tube of theselected group in place of the inert gas. Furthermore, in order toprevent harmful thermal shocks during the injection, it is desirable tobring the water of the reservoir 10 to the same temperature. Transfer ofthe water into each tube is checked by means of the external gage 13.However, in an alternative form which affords complete safety inguarding against the risk of overflow of the tube during filling withinadjacent tubes, the useful volume of the reservoir 10 is calculated inorder to be just sutficient to fill one tube.

When the tube which has thus been connected exhibits a leak, detectionof the leak is carried out immediately by determination of the presenceof hydrogen resulting from a fiow of the demineralized water within thesodium through said leak. The defective tube having thus beenidentified, it is only necessary to plug the tube, especially outsidethe end manifolds 6 and 7 after having replaced the demineralized waterwith inert gas whose pressure is then maintained at a value which isslightly higher than atmospheric pressure in order to prevent admissionof air during the plugging operation itself. The installation can thenbe rapidly restored to normal operation, only the plugged tube being outof service.

A particular case is to be contemplated when access to the differenttubes of the heat exchanger cannot be gained without discharging thesteam within the circulation tubes to the atmosphere; this is especiallythe case in which the tubes 5 extend through and have their openings intwo front plates 40 and 41 which are provided on the tank in the mannerwhich is shown in FIG. 6. An alternative form of the method accordinglyconsists in carrying the operation as follows: after completedepressurization of the steam circuit and release of the front plates 40and 41, which has the disadvantage of connecting the circuit to theatmosphere although to a very limited extent, a seal plug 42 is fittedin one end of each tube 5 whilst the other end is connected either to areservoir of the type shown in FIG. 3 or to a cylinder 43 fitted with apressure gage 44 and an expansion valve 45 for the direct admission ofhydrogen into the tube considered. The detection of hydrogen in thesodium is then carried out in the same manner as in the previous case,the remainder of the operations being identical. It should be noted thatthe advantage of employing hydrogen instead of water arises from thefact that, in this case, it is possible to operate at a lower pressurethan in the previous embodiment in which the tracer fluid isdemineralized liquid water. The sensitivity of detection of themicro-leak to be identified is thus improved to a certain extent.

As will be readily understood, the invention is not limited in any senseto the examples which have been described with reference to theaccompanying drawings but extends on the contrary to all alternativeforms.

What we claim is:

1. A method for locating a steam micro-leak within the tank of a heatexchanger having circulation tubes for water surrounded by liquid sodiumand consisting as an initial step without modifying the pressure andtemperature regime of the heat exchanger in locating and classifying theleakage in the category of micro-leaks by detection of hydrogen in thesodium then selecting from all the heatexchanger tubes a group of tubescontaining the particular tube in which the micro-leak has developed,then bringing the heat exchanger to a constant temperature at which thethermal power exchanged between the sodium and water in the tubes issubstantially zero, simultaneously depressuring the steam within all thetubes of the heat exchanger to a pressure which is slightly higher thanthe pressure of the sodium, connecting each tube of the groupsuccessively to a reservoir containing hydrogenated fluid, introducingsaid fluid into said tubes at a pressure equal at a maximum to thepressure of the steam under normal operating conditions and at thetemperature of the sodium, detecting the presence of hydrogen in thesodium, selecting the tube in which a leak has developed and thenplugging said tube externally of the tank.

2. A method according to claim 1 including the step, prior tointroduction of the hydrogenated fluid into each of the tubes of theselected group, of filling said tubes with an inert gas at the pressureof the steam prior to depressurization of the heat exchanger.

3. A method according to claim 2 including the step, prior to pluggingof the tube which exhibits a leak in the selected group, of replacingthe hydrogenated fluid introduced into said tube by the inert gas, saidgas being brought to a pressure slightly higher than atmosphericpressure.

4. A method according to claim 1, including the step, duringintroduction of the hydrogenated fluid into each tube of the selectedgroup, of reducing the rate of flow of sodium within the heat-exchangertank to increase the sensitivity of detection of the hydrogen throughthe leak.

5. A device for locating a steam micro-leak within the tank of a heatexchanger having circulation tubes for water surrounded by liquid sodiumincluding an external reservoir for hydrogenated fluid, means forvarying the pressure and the temperature of said fluid, means forintroducing said fluid into said tubes, and means for determining themass of fluid introduced successively into each of the tubes of aselected group of said tubes.

6. A device according to claim 5, wherein the hydrogenated fluid isdemineralized and degassed water.

7. A device according to claim 6, including a heating element bringingthe demineralized and degassed water to the isothermal temperature ofthe sodium within the tank said heating element being in contact withthe wall of said reservoir.

8. A device according to claim 6, the volume of said reservoir beingsubstantiall equal to the volume of a heat-exchanger tube.

9. A device according to claim 5, wherein the hydrogenated fluid issteam.

10. A device according to claim 5, wherein the hydrogenated fluid ishydrogen gas.

11. A device according to claim 5, the hydrogenated fluid liberatinghydrogen by chemical reaction with sodium.

12. A device according to claim 5, said reservoir ineluding a dischargepipe, means for connecting said pipe successively to each tube of theselected group of tubes including an opening in the lateral wall of eachtube and a detachable pipe-coupling in said opening and a nonreturnvalve in said pipe-coupling.

13. A device according to claim 12, each tube of the selected group oftubes including means for temporarily shutting-01f said opening.

14. A device according to claim 12, said discharge pipe being connectedin parallel with a supply of inert gas under pressure.

15. A device according to claim 5, said reservoir being acompressed-hydrogen cylinder connected successively to one extremity ofeach tube of the selected group of said tubes through a front plate inthe Wall of said tank, and a seal plug closing the other tube-extremityat a second front plate in the wall of said tank.

References Cited UNITED STATES PATENTS 2,948,516 8/1960 Martinelli et al7340 X 3,122,668 2/1964 Cuny 7340 X 3,400,753 9/1968 Slover 73-405 R XLOUIS R. PRINCE, Primary Examiner W. A. HENRY II, Assistant Examiner

